LUCENE-9950: New facet counting implementation for general string doc value fields

lucene/CHANGES.txt

@@ -41,6 +41,8 @@ New Features
 * LUCENE-9507: Custom order for leaves in IndexReader and IndexWriter
   (Mayya Sharipova, Mike McCandless, Jim Ferenczi)
 
+* LUCENE-9950: New facet counting implementation for general string doc value fields
+  (SortedSetDocValues / SortedDocValues) not created through FacetsConfig (Greg Miller)
 
 Improvements
 ---------------------

lucene/facet/src/java/org/apache/lucene/facet/StringDocValuesReaderState.java

@@ -0,0 +1,78 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.lucene.facet;
+
+import java.io.IOException;
+import java.util.List;
+import org.apache.lucene.index.DocValues;
+import org.apache.lucene.index.IndexReader;
+import org.apache.lucene.index.LeafReaderContext;
+import org.apache.lucene.index.OrdinalMap;
+import org.apache.lucene.index.SortedSetDocValues;
+import org.apache.lucene.util.packed.PackedInts;
+
+/**
+ * Stores an {@link OrdinalMap} created for a specific {@link IndexReader} ({@code reader}) + {@code
+ * field}. Enables re-use of the {@code ordinalMap} once created since creation is costly.
+ *
+ * <p>Note: It's important that callers confirm the ordinal map is still valid for their cases.
+ * Specifically, callers should confirm that the reader used to create the map ({@code reader})
+ * matches their use-case.
+ */
+public class StringDocValuesReaderState {
+
+  final IndexReader reader;
+  final String field;
+  final OrdinalMap ordinalMap;
+
+  /**
+   * Construct state specific to a reader + field. This builds an {@link OrdinalMap} that can be
+   * reused for mapping segment-specific ordinals to global ordinals for the given field. Keep in
+   * mind that the state is only valid for the specified {@link IndexReader}, so opening new readers
+   * (e.g., to pickup NRT updates) requires constructing a new state instance.
+   */
+  public StringDocValuesReaderState(IndexReader reader, String field) throws IOException {
+    this.reader = reader;
+    this.field = field;
+    ordinalMap = buildOrdinalMap(reader, field);
+  }
+
+  private static OrdinalMap buildOrdinalMap(IndexReader reader, String field) throws IOException {
+    List<LeafReaderContext> leaves = reader.leaves();
+    int leafCount = leaves.size();
+
+    if (leafCount <= 1) {
+      return null;
+    }
+
+    SortedSetDocValues[] docValues = new SortedSetDocValues[leafCount];
+    for (int i = 0; i < leafCount; i++) {
+      LeafReaderContext context = reader.leaves().get(i);
+      docValues[i] = DocValues.getSortedSet(context.reader(), field);
+    }
+
+    IndexReader.CacheHelper cacheHelper = reader.getReaderCacheHelper();
+    IndexReader.CacheKey owner = cacheHelper == null ? null : cacheHelper.getKey();
+
+    return OrdinalMap.build(owner, docValues, PackedInts.DEFAULT);
+  }
+
+  @Override
+  public String toString() {
+    return "StringDocValuesReaderState(field=" + field + " reader=" + reader + ")";
+  }
+}